(Co)polymers useful in fuel cell membranes and other acidic, high-temperature, hydrated environments, must contain only stable functional groups. Unfortunately, most polymer structures, and their precursor monomers, contain groups that do not withstand such environments, for example ester, acrylamide, and many aliphatic functional groups. To solve this problem, a novel approach to monomer and polymer syntheses is required using only hydrolytically-stable and peroxide-stable functional groups. In addition, the starting materials should be of low-cost, the process complexity kept minimal, and the final structures well-defined.
Typical polyelectrolytes (as disclosed in U.S. Pat. No. 7,396,880) are styrenic-type polyelectrolytes with pendent aromatic groups, but no aromatic groups in the polymer backbone. They can be synthesized and incorporated into a PVDF/polyelectrolyte blend membrane. Membranes fabricated using these methodologies have shown dramatically improved hydrolytic stability in a high-temperature (80° C.), acidic (pH<1) environment, versus prior generations of materials. However, to increase cell performance and reduce balance-of-plant costs for an operating fuel cell, the target operating conditions for fuel cell membranes are becoming increasingly more severe. One important requirement is to have a membrane which will operate and remain very stable at temperatures in excess of 80° C. The materials described in U.S. Pat. No. 7,396,880 have proven to perform well at 80° C., but tend to show degradation when used at higher temperatures. These degradation pathways include, but are not limited to, peroxide attack on susceptible functionalities or positions on the polyelectrolyte, and loss of sulfonate groups through aromatic-ring desulfonation. The mode of degradation is likely due to scission of the carbon-carbon, hydrogenated aliphatic backbones, common to all styrenic-type (co)polymers. The benzylic hydrogens which are bonded to carbons adjacent to the aromatic rings are known to be particularly prone to attack by adventitious peroxide species (Scheme 1). The hydrogen atoms can be relatively easily removed from the carbon-carbon backbones, generating a carbon-centered radical on the polymer chain. This radical can then participate in one of a number of processes either causing a release of the aromatic ring from the polymer backbone (Scheme 2) or a scission of the polymer chain (Scheme 3). Both of these processes are undesirable as they cause loss of the active sulfonate groups from the polyelectrolytes or reduction in overall molecular weight, respectively.



The degradation pathways described herein are fundamentally different from those described in U.S. Pat. No. 7,396,880, where, a specific functional group was susceptible to hydrolytic attack by acid and water, causing predominantly a loss of sulfonate groups. Under 80° C. operation, this rate of this hydrolytic mechanism was significantly more rapid than any other degradation pathway. The polyelectrolytes described here (Schemes 1-3), and those described in U.S. Pat. No. 7,396,880, ameliorated the hydrolytic degradation and permitted 80° C. operation of membranes containing such polyelectrolytes. Given this increased chemical stability, the additional degradation mechanisms described above are becoming more predominant than previously recognized. The fundamental weak component to these polyelectrolytes is their aliphatic backbone, particularly, benzylic positions which are present in styrenic-type copolyelectrolytes.